1. Field of the Invention
The present invention relates to a four-cycle internal combustion engine and, more particularly, relates to a high-revolution type four-cycle internal combustion engine mounted on a vehicle such as a sport type motorcycle.
2. Description of Related Art
In a four-cycle internal combustion engine used as a power source for a vehicle such as a motorcycle, a valve system is typically provided. The valve system typically includes a valve for opening/closing an intake port and an exhaust port (collectively referred to hereinafter as a “port”) at a prescribed timing, and a coil valve spring for urging the valve in the direction that the port is closed.
A method for inhibiting abrasion of a valve spring in such an engine is known, in which spring seats are provided on both top and bottom ends of the valve spring in a contact manner. See, for example, JP-U-Hei 3-87905 (pages 4-6 and FIG. 1).
However, such valve systems have experienced problems in the case of high revolution speed in the engine (specifically, a crankshaft). In such high revolution speed, the speed of reciprocating motion of a valve is also high. If the speed of reciprocating motion of the valve increases, a valve spring pushed down by a locker arm (or a lifter) cannot contract straight, but instead contracts deformingly in the lateral direction (short side direction). Therefore, smooth operation of the valve is hindered and durability of the valve system is lowered.